1. Field of the Invention
The present invention relates generally to the field of computers, and more particularly, to a system and method of managing read resources in a multiprocessing environment.
2. Related Art
Modern multiprocessing systems typically have a plurality of processors, a main memory and other input/output (I/O) devices that are coupled to a main system bus. To permit maximum utilization of the bus these systems use split transactions. (For simplification, the processors, main memory and I/O devices will be referred to as "nodes".) A split transaction is a read transaction that is split into a read request transaction that contains the address of the data requested, and a reply transaction that contains the requested data. Reply transactions are also called read responses. Each transaction must be tagged so that the requesting node and the replying node (e.g., main memory or a processor having the requested data in its cache) can keep track of the status of the read request. When individual transactions being performed by a node are long in duration, a split transaction approach allows several transactions to simultaneously occupy the bus, thus increasing the effective bandwidth of the bus.
In order to support split transactions on a shared bus, both the requesting node and the replying node must arbitrate for the bus to perform their respective functions. Thus a split transaction bus has higher bandwidth, but usually has higher latency than a bus that is held for the entire read transaction.
A cache coherent system bus can also use split transactions. However, to maintain cache coherency all nodes on the bus must be aware of all pending split transactions. Those nodes that do not have caches must also be aware of all pending split transactions. What is desired is a mechanism to handle split transactions and maintain cache coherency across a multiprocessing domain.